1. Field of the Invention
The invention relates to a process for producing a coating for protecting superalloy articles against high-temperature oxidation and hot corrosion, a protective coating produced by such a process, and superalloy articles protected by the coating. The invention is applicable in particular to the protection of hot superalloy parts of turbomachines.
2. Summary of the Prior Art
For more than 30 years the manufacturers of turbine engines for both land and aeronautical use have been addressing demands for increased turbomachine efficiency, and reduction of specific fuel consumption and polluting emissions of the COX, SOX and NOX types as well as unburnt constituents. One way of meeting these demands is to study combustion fuel stoichiometry and thus increase the temperature of the gases issuing from the combustion chamber and impacting the first turbine stages. The materials used for the construction of the turbine must therefore be made compatible with these increased combustion gas temperatures. One solution is to develop a refractory nature for the materials used in order to increase the maximum working temperature and the working life in terms of creep and fatigue. This solution became widely used following the appearance of nickel and/or cobalt superalloys, and has undergone a considerable technical advance in the change from equiaxial superalloys to monocrystalline superalloys (a creep gain of 80 to 100.degree. C.).
Another important development in turbine technology is connected to the new sales and guarantee practices in this field. The usual practice is for the customer to be given guarantees for the working lives of land and aeronautical turbines. It is therefore of considerable economic interest to a manufacturer of turbine engines to achieve a significant increase in the working life of the engine components, and particularly the components of the "hot" parts.
This raises the problem of increasing the protection of hot-part components against high temperature oxidation (T&gt;approximately 950.degree. C.) and hot corrosion (at intermediate temperatures in the presence of SO.sub.2 /SO.sub.3 and deposits of melted sulphate and/or vanadate type salts).
There are two main categories of coatings for protecting superalloys against high-temperature oxidation and hot corrosion, these being simple coatings of aluminides and their derivatives, and alloy coatings.
Coatings belonging in the category of simple aluminides and their derivatives basically consist of a nickel aluminide alloy, NiAl, comprising an atomic percentage of aluminum between 40 and 55%. As a result of oxidation at high temperature this type of alloy forms a protective layer of aluminum oxide limiting interaction between the coating and the environment (oxygen, melted salts, SO.sub.2 /SO.sub.3). These coatings can be deposited thermochemically by pot cementation or by vapour phase cementation. They can also be obtained by the deposition of an aluminizing paint followed by appropriate annealing. The main advantage of these coatings is simplicity of implementation, low production costs and the possibility of providing articles of complex shape with uniform coatings.
However, the performance of coatings of this type is limited. At high temperatures the alumina formed is stressed and adheres unsatisfactorily. It exfoliates readily during thermal cycling, leading to aluminum consumption and depletion in the outer part of the coating. This consumption seriously limits the working life of the coating, which provides very little protection once the aluminum reserve has been used up. As regards hot corrosion, the pure alumina layer formed may be dissolved by interaction with environments of melted sulphate salts or a mixture of sulphate and vanadate salts.
One good way of significantly increasing the working life of these coatings is to modify the simple aluminide NiAl by various elements such as chromium and/or some platinum group precious metals. The coating operation then takes the form of making an initial deposit of each modifying metal on the superalloy article, followed by an aluminization. In some cases a specific heat treatment is effected between the step of initial deposition of the modifying metal and the actual aluminization step.
The use of chromium as a modifying metal is described, for example, in French patent 2559508, wherein the chromium is applied thermochemically. The main function of the chromium is to limit the acidity or basicity of the melted salts in hot corrosion conditions by the dissolution of cations acting as an acido-basic buffer in the melted salt.
The use of platinum as a modifying metal is described in French patent 2018097. In this case the platinum is deposited electrolytically on the superalloy article. This precious metal is present in considerable proportions in solid solution in the .beta.-NiAl phase of the nickel aluminide. It improves the adhesion of the protective alumina layer (cyclic oxidation) and also confers good resistance to the environment in the presence of melted salts (hot corrosion).
An alternative to the use of platinum as a modifying metal for simple aluminide coatings is to replace it by palladium. As French patent 2638174 teaches, the resulting coatings have a resistance to oxidation and hot corrosion which is equivalent to that of platinum-modified aluminides at a much lower cost.
Unlike coatings of simple aluminides and their derivatives, alloy coatings are not obtained by procedures involving high-temperature diffusion between the superalloy substrate and the coating during preparation. On the contrary, these coatings involve depositing on the substrate an already formed alloy of a composition suitable for the required purpose, such as resistance to oxidation and hot corrosion.
The alloy coatings most commonly used for high temperature protection of superalloy substrates are coatings of the MCrAlY type. In these coatings the symbol M represents the alloy base, which may be cobalt, nickel or iron, or a combination of two or more of these three metals. The chromium is present in a proportion of between 10 and 40% by weight, and serves mainly to increase the hot corrosion resistance of the coating. The aluminum is present in a proportion of between 2 and 25% by weight, its main function being the hot formation of a protective alumina layer which is required to be of slow growth, as chemically stable as possible to withstand hot corrosion, and to be very adhesive so as to withstand differential expansion stressing during high-temperature thermal cycling. Yttrium (Y) is present in proportions between a few tens of ppm and a few % by weight, and has two functions. Firstly, it can trap the residual sulphur of the alloys in the form of very stable sulphides and thus prevent the residual sulphur from hot diffusion towards the oxide/coating interface, where it tends to segregate and thus greatly limit the adhesion of the alumina layer. Secondly, it is incorporated in the form of mixed yttrium and aluminum oxides at the grain junctions of the alumina layer formed. These mixed oxides modify the diffusion mechanisms in the alumina to lead to the formation of an alumina free from residual growth stresses and therefore one which sticks much better to the coating. In general, yttrium is a powerful promoter of adhesion between the coating and the oxide in MCrAlY coatings.
Some other elements such as hafnium, zirconium, cerium, lanthanides and, in general, most of the rare earths can play a role very similar to that of yttrium as regards the adhesion of the protective alumina layers. Also, the contribution of yttrium and related elements, sometimes called active elements, to the effectiveness of the protective coatings of superalloys is limited solely to high-temperature oxidation. It has not been possible to show any active element effect in the case of hot corrosion of the coated superalloys.
Alloy coatings can be deposited by techniques such as:
Thermal plasma projection in air, in vacuo or in a controlled atmosphere; PA1 HVOF (high velocity oxygen fuel) thermal projection and other thermal projection processes; PA1 Detonation gun; PA1 Explosion plating; PA1 Electron bombardment evaporation; PA1 Multi-arc plasma evaporation; and, PA1 Cathodic sputter techniques. PA1 (a) Deposition of MCrAlY alloy by thermal projection; PA1 (b) Optional thermochemical chromizing; PA1 (c) Thermochemical aluminization; PA1 (d) Electrolytic deposition of platinum; and, PA1 (e) Diffusion heat treatment of the platinum deposit in the external part of the aluminized MCrAlY coating. PA1 a) making a first deposit of a powdered alloy containing at least chromium, aluminum and an active element on the article to be coated such that said first deposit has a residual open porosity; PA1 b) making a metallic electrolytic second deposit containing at least one platinum group metal on said first deposit so as to fill said residual open porosity of said first deposit; and, PA1 c) carrying out a heat treatment to effect interdiffusion between the powder first deposit and said electrolytic second deposit whereby said platinum group metal is present throughout the thickness of the protective coating. PA1 a) making a first deposit of a powdered alloy containing at least chromium, aluminum, an active element and at least one platinum group metal on the article to be coated such that said deposit has a residual open porosity; and PA1 b) aluminizing said deposit so as to enrich the coating in aluminum and fill said residual open porosity.
All these techniques share a number of major disadvantages. They are costly to use, there are difficulties in controlling deposit quality, and there are difficulties in controlling the deposition of MCrAlY on articles having complex shape since the techniques are directional and cannot form a uniform coating on articles of complex shape.
As alternatives to using a coating of either of the two categories described above, a number of solutions have been developed.
A first solution is aluminized MCrAlY coatings. Pack or vapour phase aluminization on a MCrAlY coating deposited by one of the techniques already described has the advantage of obtaining an aluminum-enriched external composition of the coating, so that the working life thereof is prolonged, particularly in high temperature oxidation conditions. However, this solution is not very much better than a conventional MCrAlY application and suffers from the same limitation as regards cost and control of uniformity on articles of complex shape.
A second solution consists of electrophoretic MCrAlY coatings. The process for making this type of coating is described, for example, in French patent 2529911. The process comprises depositing a coating consisting of agglomerated powders of MCrAlY alloys on a nickel-based superalloy substrate by an adapted electrophoresis technique. Since this porous deposition has no mechanical strength the porosity must be filled by vapour phase aluminization. Aluminization serves to consolidate and fill the pores left between the agglomerated MCrAlY powder grains, and the final structure is very similar to that of a traditional aluminized MCrAlY coating.
The non-directional electrophoresis technique makes it possible to achieve uniform coatings on articles of complex shape, such as turbine distributor vane doublets. For an equivalent quality of protection this technique is much cheaper than a MCrAlY coating deposited by plasma and then aluminized. However, the resulting coating performs little better than a coating of MCrAlY on its own.
A third solution involves the combination of a plasma-deposited MCrAlY coating and a precious metal as described, for example, in EP-A-0587341. The coating is obtained by a process comprising the following steps:
A major disadvantage of this coating is that it contains platinum only in its outer region. In fact the coating consists of a conventional MCrAlY coating and a superposed platinum modified aluminide coating. The beneficial effects of MCrAlY coatings and platinum-modified aluminide coatings are juxtaposed but are not additive. Synergy of the effect cannot therefore be achieved. Also, the total thickness of the coating is at least 100 .mu.m, so that the extra weight may cause problems if the coating is used on rotating members. Furthermore, a coating of this kind is very expensive, its cost being at least equal to the sum of the costs of a traditional MCrAlY coating and of a platinum-modified aluminide coating. Finally, there is still an acute problem in coating objects of complex shape.